The best way to build diffs are very simple if you do the right things.
1. Take diff cases either new or when rebuilding the first time on the flat side and rub them in a figure 8 motion on a flat surface on #400 emery cloth and then clean them . This will make the cases perfectly flatt.
2. Build the diff as instructed and fill them to 80%.
3. Place some silicone gasket sealent on the filled diff case. Then place the gasket on the diff case, line up the holes properly and give it about 10 minutes to set. Then place the large bevel gear (front or rear diffs) or spur gear (center diff) and tighten it down on the opposite rotating method. Just make the screws good and tight. There is no need to over tighten as you'll just strip the diff cases. Then take some of the silicone gasket sealent and place some over the head of the screws to cover them and some also on the small set screw. This silicone sealent is easily removed upon your next rebuild.
4.Then you shim your diff properly in the bulkheads as instructed. Then place some grease (I use AE black grease) on the bottom of the bulkheads before attaching them to the chassis. This will stop dirt from entering the diff area t and keep the bevel and pinion gears in much better condition and the internal clean.
5. Some diffs need to be reshimmed after some run in. Again go back to steps 2,3,4 and you should be set to go.
Your diffs should not leak if built properly and you should be able to get at least 2-3 gallons of fuel through them before the diff oils need cleaning and a rebuild is necessary.